Buffers are processed in the order specified. The readv() function works just like
read(2)
except that multiple buffers are filled.

The writev() function works just like
write(2) except that multiple buffers are written out.

RETURN VALUE

On success, the readv() function returns the number of bytes read; the writev() function returns the number of bytes written.
On error, -1 is returned, and errno is set appropriately.

ERRORS

The errors are as given for read(2)
and write(2).
Additionally the following error is defined:

Tag

Description

EINVAL

The sum of the
iov_len values overflows an
ssize_t value. Or,
the vector count count is less than zero or greater than the
permitted maximum.

CONFORMING TO

4.4BSD (the readv() and writev() functions first appeared in 4.2BSD), POSIX.1-2001.
Linux libc5 used size_t as the type of the count parameter,
and int as return type for these functions.

LINUX NOTES

POSIX.1-2001 allows an implementation to place a limit on
the number of items that can be passed in
vector. An implementation can advertise its limit by defining
IOV_MAX in
<limits.h> or at run time via the return value from
sysconf(_SC_IOV_MAX). On Linux, the limit advertised by these mechanisms is 1024,
which is the true kernel limit.

However, the glibc wrapper functions do some extra work if
they detect that the underlying kernel system call failed because this
limit was exceeded. In the case of
readv() the wrapper function allocates a temporary buffer large enough
for all of the items specified by
vector, passes that buffer in a call to
read(), copies data from the buffer to the locations specified by the
iov_base fields of the elements of
vector, and then frees the buffer.

The wrapper function for writev() performs the analogous task using a temporary buffer and a call to
write().

BUGS

It is not advisable to mix calls to functions like
readv() or writev(), which operate on file descriptors, with the functions from the stdio
library; the results will be undefined and probably not what you want.